Trends in the Periodic Table

The two big ideas

1. Across a period (left to right): the number of protons goes up, but the electrons are added to the same shell. The pull on the outer electrons gets stronger.
2. Down a group: a new shell is added each time. The outer electrons are further away, and the inner shells shield them from the nucleus. The pull gets weaker.

Atomic radius (size)

• Down a group: atoms get bigger, because there are more shells.
• Across a period: atoms get smaller, because more protons pull the same shell in tighter.

Sodium is bigger than chlorine, even though chlorine has more electrons.

Reactivity of metals (group 1)

Metals react by losing electrons. The easier it is to lose the outer electron, the more reactive the metal.

Down group 1, reactivity increases. Lithium fizzes gently on water. Sodium whizzes around and melts. Potassium bursts into a lilac flame. Caesium explodes. The outer electron is further away and easier to lose each time.

$$\text{lithium}+\text{water}\to \text{lithium hydroxide}+\text{hydrogen}$$

Reactivity of non-metals (group 17)

Non-metals react by gaining electrons. The stronger the pull on an incoming electron, the more reactive.

Down group 17, reactivity decreases. Fluorine is the most reactive element of all. Iodine is much calmer. In bigger atoms the incoming electron is further from the nucleus and feels a weaker pull.

A more reactive halogen displaces a less reactive one from its salt:

$$\text{chlorine}+\text{potassium bromide}\to \text{potassium chloride}+\text{bromine}$$

Metallic character

Metallic character is how much an element behaves like a metal.

• It increases down a group. Carbon at the top of group 14 is a non-metal, silicon and germanium are metalloids, tin and lead are metals.
• It decreases across a period. Period 3 goes from metals (Na, Mg, Al) to a metalloid (Si) to non-metals (P, S, Cl, Ar).

Other trends to know

Electronegativity is how strongly an atom pulls on shared electrons in a bond. Fluorine is the most electronegative element. Noble gases are usually left out because they rarely bond.

Quick summary

• Trends depend on the number of protons and on how far the outer electrons are from the nucleus.
• Atoms get bigger down a group and smaller across a period.
• Group 1 metals get more reactive down the group. Group 17 halogens get less reactive down the group.
• Metallic character increases down and decreases across.
• Electronegativity increases across and decreases down. Fluorine is the highest.

Quiz